Merge branch 'stable-2.16' into stable-3.0

* stable-2.16:
  Bump Bazel version to 2.2.0
  Bump required Bazel version to 2.1.0
  Upgrade bazlets to latest stable-2.16 to build with 2.16.16 API
  Documentation: Add missing footer that links to plugin index page
  Documentation: Refactor to match other plugin documentation
  Add an empty tools/BUILD file explicitly for Bazel
  Upgrade bazlets to latest stable-2.16
  Upgrade bazlets to latest stable-2.16
  Upgrade bazlets to latest stable-2.15
  Upgrade bazlets to latest stable-2.14

Change-Id: Ia85129611b6f4b110e2b298c108d23668b1908a5